The exemplary embodiments relate generally to electronic components and more specifically to a circuit board assembly having one or more heat pipes to enhance cooling of the circuit board.
In avionics and other applications, printed circuit boards (“PCBs”) are commonly mounted within a chassis. The heat generated by electronic devices associated with the PCB may be dissipated by transfer to a metal wall of the chassis through the PCBs. The heat may then be sent to an external heat sink by conduction through the chassis wall, and is finally taken away by either cool air circulating about the heat sink or a cold plate. Because of high thermal resistance in the heat transfer path, the temperature generally increases with continued operation until steady state is reached. This leads to a larger temperature gradient between the electronic devices that are generating heat and the heat sink. This larger temperature gradient may adversely affect the performance of the electronic devices.
Heat pipes have been used to assist in the transfer of heat from the PCBs. A typical heat pipe may be made of a sealed hollow tube. The tube may be made of a conductive metal such as copper or aluminum. The tube contains a relatively small quantity of a fluid (such as water, ethanol or mercury) with the remainder of the tube being filled with the vapor phase of the fluid, all other gases being excluded. Disposed within the tube, a wick structure exerts a capillary force on the liquid phase of the fluid. This may typically be a sintered metal powder or a series of grooves parallel to the tube axis, but it may be any material capable of soaking up the coolant. If the heat pipe is placed in an arrangement so that it has a continual slope with the heated end down, no inner lining is needed. The working fluid simply flows with gravity back down the tube.
Heat pipes employ evaporative cooling to transfer thermal energy from one point to another by the evaporation and condensation of the fluid. Heat pipes rely on a temperature difference between the ends of the pipe, and cannot lower temperatures at either end beyond the temperature of the cool end. When one end of the heat pipe is heated the fluid inside the pipe at that end evaporates and increases the vapor pressure inside the cavity of the heat pipe. The latent heat of evaporation absorbed by the vaporization of the fluid removes heat from the hot end of the pipe. The vapor pressure at the hot end of the pipe is higher than the equilibrium vapor pressure at the cooler end of the pipe, and this pressure difference drives a rapid mass transfer to the condensing end where the vapor condenses, releases its latent heat, and transfers heat to the cool end of the pipe. The condensed fluid then flows back to the hot end of the pipe. In the case of vertically-oriented heat pipes, the fluid may be moved by the force of gravity. In the case of heat pipes containing wicks, the fluid may be returned by capillary action.
As PCB performance and power increase, so does the temperature generated by the PCBs. The largest increases in temperature may occur between the chassis wall and the PCB edge, and between the PCB edge and the PCB center. Previous assemblies have not been adequate in dissipating the increased heat generated by higher power PCBs. These assemblies have conducted heat away through poor conductive paths such as the circuit board itself, and/or through clamping devices that secure the assemblies to the chassis.